The present invention relates to micro-electromechanical die modules of the type having a planarized, patterned thick film layer sandwiched between silicon substrates, and more particularly to an improved thermal ink jet die module for use as a printhead and method of manufacture therefor, the die module eliminating the effects of standoff between two bonded parts thereof caused by topographic formations formed in a thick film insulating layer sandwiched between said two parts during deposition and patterning thereof. The ink jet die module is a specific example of a general class of micro-electromechanical die modules which combine electrical and mechanical functionality in an integrated device.
In existing thermal ink jet printing systems, an ink jet printhead expels ink droplets on demand by the selective application of a current pulse to a thermal energy generator, usually a resistor, located in capillary-filled, parallel ink channels a predetermined distance upstream from the channel nozzles or orifices. U.S. Pat. No. Re. 32,572 to Hawkins et al. exemplifies such a thermal ink jet printhead and several fabricating processes therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes (heater plate), and the second part is a substrate having at least one recess anisotropically etched therein to serve as an ink supply reservoir when the two parts are bonded together (channel plate). A linear array of parallel grooves are also formed in the second part, so that one end of the grooves communicate with the reservoir recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be made simultaneously by producing a plurality of sets of heating element arrays with their addressing elements on a silicon wafer and by mating a second silicon wafer having a corresponding plurality of sets of channel grooves and associated manifolds therein. After the two wafers are aligned and bonded together, the mated wafers are diced into many separate printheads.
Improvements to such two-part, thermal ink jet printheads include U.S. Pat. No. 4,638,337 to Torpey et al., that discloses an improved printhead similar to that of Hawkins et al., but has each of its heating elements located in a recess (termed heater pit). The recess walls containing the heating elements prevent lateral movement of the bubbles through the nozzle and, therefore, the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation. In this patent, a thick film insulative layer such as polyimide, Riston.RTM. or Vacrel.RTM. is formed on the wafer containing the heating elements and patterned to provide the recesses for the heating elements, so that the thick film layer is interposed between the two wafers when they are mated together. U.S. Pat. No. 4,774,530 to Hawkins further refines the two-part printhead by disclosing an improvement over the patent to Torpey et al. In this patent, further recesses (termed bypass pits) are patterned in the thick film layer to provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the groove closed ends to the manifold recess. The heater plates, having the aforementioned improvements of heater pits and bypass pits formed in the thick film insulative layer covering the heater plate surface, are aligned with and bonded to the channel plate, so that each channel groove has a recessed heating element therein and a bypass pit to provide an ink passage from the ink manifold to the channel groove.
Thorough bonding between heater and channel plates is paramount to maintaining the printing efficiency, droplet size consistency, and operational reliability of an ink jet printhead. U.S. Pat. No. 4,678,529 to Drake et al. discloses a method of bonding ink jet printhead components together by spin coating or spraying a relatively thin, uniform layer of adhesive on a flexible substrate and then manually placing the flexible substrate surface with the adhesive layer against the channel wafer surface having the etched sets of channel grooves and associated manifolds or reservoirs. A uniform pressure and temperature is applied to ensure adhesive contact with all coplanar surface portions and then the flexible substrate peeled away, leaving a uniformly thin coating on the channel wafer surface to be bonded to the heater wafer. A more mechanized process to place the adhesive coating on the channel wafer without manual operator involvement and consequent variation in the amount of adhesive layer transferred to the channel wafers, especially in the thickness variations from wafer-to-wafer, is described in U.S. Pat. No. 5,336,319, to Narang et al. The prior art process for bonding die modules may work well at 300 dpi, but as printhead resolution increases, a number of problems arise.
Although advances have improved the thickness uniformity of the adhesive layer which bonds the ink jet printhead heater and channel plates, insufficient adhesion between bonded heater and channel plates causes a host of problems affecting high resolution printhead operation, such as, for example, different drop sizes between adjacent channels, because unwanted protruding topographical formations or lips are formed in the thick film layer during the patterning and curing of the heater pits and bypass pits. These topographical formations prevent adequate contact between the channel wafer surface with the adhesive layer and the thick film layer on the heater wafer. Since increased adhesive layer thickness is not a practical solution because it tends to spread or wick into the channels, the inter-channel gaps between bonded heater and channel plates should be eliminated in order to insure consistent printhead firing characteristics. As taught by the above identified U.S. patents, two wafers are bonded together after alignment for subsequent dicing into individual printheads. Each printhead part is formed individually on two separate substrates or wafers, where one contains heating elements and the other ink channels or passageways. The wafer containing the ink channels is silicon, and the channels are formed by an anisotropic etching process. The anisotropic or orientation dependent etching has been shown to be a high yielding process that produces very planar and highly precise channel plates. The other wafer containing the heating elements as well as heater addressing logic is covered by a thick film insulating layer in which heater and bypass pits are formed using photolithography. The thick film-layer is preferably polyimide, because it can be patterned in the geometries required, can withstand the temperature cycling of the heater, and is chemically resistant to the ink. However, one drawback with the polyimide material is its tendency to form unwanted topographical formations, such as raised edges or lips (1-8 microns high) at photoimaged edges. When bonding both heater and channel plates together, a standoff between the two plates is caused by the raised edges, which reduces the adhesiveness of the bond between the two plates and which cause the formation of inter-channel gaps.
In roofshooter type thermal ink jet printheads, such as disclosed in U.S. Pat. No. 4,789,425 to Drake et al., each printhead is composed of parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes (heater plate). This part has a thick film insulative material deposited on the surface with the heating elements and addressing electrodes, and the thick film layer is photolithographically patterned to form ink flow paths, each containing a one of the heating elements, from an ink inlet. This inlet is usually provided through the flat substrate or heater plate to the heating elements. This patterned thick film layer is usually referred to as a "barrier layer". The final part is a nozzle plate containing an array of nozzles. The nozzle plate is aligned and bonded to the patterned barrier layer, so that each nozzle is aligned directly over one of the heating elements for droplet ejection through the nozzles in a direction perpendicular to the heating element. Thus, the roofshooter type thermal ink jet printhead is also concerned with topographic formation in the surface of the patterned barrier layer which would prevent adequate bonding of the nozzle plate thereto.
Polyimide topography, such as raised edges or lips, are undesirable byproducts resulting from photoimaged and cured heater pits and bypass pits or trenches on heater plates. The raised edges are polyimide topographical features that are formed at the edge of photoimaged areas that do not shrink during curing as would the generally non-patterned larger areas of the polyimide. Consequently, raised edges critically interfere with both the mating and bonding of the heater and channel plates of edge shooter type printheads and the mating and bonding of the heater and nozzle plates of the roofshooter printheads.
Another form of polyimide topography is encountered in the form of edge beads or raised areas at the edge of the wafer, when a layer of liquid polyimide is dispensed and spun onto a wafer. When the contact area on the wafer is incapable of spreading further due to the contact angle at the edge of the wafer, centripetal forces push the spinning liquid polyimide towards the outside of the wafer to form an edge bead. The edge bead on a 4 inch diameter wafer, for example, is on the order of 3 mm-15 mm wide radially from the outer edge thereof. Because the wafers generally have chordal portions removed (called "flats") to provide straight edges for subsequent use in identifying wafer type, crystal plane orientation, as well as for alignment features in assembly or fabrication jigs, the periphery of the wafers is not completely circular. Thus, the thickness of the edge bead varies from a few micrometers thicker than the rest of the polyimide layer to twice as thick as the majority center portion. Due to the asymmetry of the periphery of the wafer caused by the flats, the thickness of the edge bead varies substantially around the edge of each wafer. Such edge beads of polyimide prevent adequate bonding between the wafers. Edge beads can also cause a reduction in yield, because the additional stress placed on the center area of the channel plate during heater and channel plated bonding may cause cracking. Edge beads, if removed from the edge of the heater wafers, cantilevers the channel plate at its outside edges and can again cause cracks to be formed in the outer peripheral area of the channel wafer. Such cracking in the channel wafer will degrade the reliability of the individual printheads after they have separated from the wafer pair.
Raised edges and edge beads, however, are not the only topographical formation created from photoimaged polyimide. Other topographical formations, such as wall sags or dips, compound the negative effects of raised edges by adding to the standoff between the bonded heater and channel plates. Wall dips are slumps in the polyimide walls between closely adjacent polyimide photoimaged pits. The polyimide layer sandwiched between the two wafers generally has a thickness of 10 to 40 .mu.m (cured) and can form more than 2 microns of topographical variation. The bonding adhesive is approximately 2 microns or less thick which does not allow the adhesive to bridge or fill in the formation of inter-channel gaps caused by the topographic formations. These inter-channel gaps can allow crosstalk between channels when drops are being ejected. As the patent '529 to Drake et al. teaches, care must be taken when applying adhesive in bonding the channel and heater plates so as to insure all surfaces in contact with the ink are free of adhesive, in order that the ink channels are not obstructed during operation.
A final cause of polyimide surface topography results from the presence of topography associated with the microelectronic device fabrication prior to spin casting the polyimide. Spin casting tends to cause the polyimide to conform and replicate features present on the wafer's surface. Since the surface contains features up to 4 .mu.m thick, the polyimide surface varies by a similar amount. It is important to point out that even if no polyimide was present, it would still be difficult to completely bond a channel wafer to a heater wafer. In this content it is desirable to add an intermediate polyimide layer, if its surface can subsequently be planarized, after first being patterned to expose critical device structures. In the more general case of microelectromechanical die modules, the polyimide layer or other suitable organic layer can be added solely for this purpose.
One method of minimizing heater and channel plate standoff of printheads using a modified printhead fabrication sequence is disclosed in U.S. patent application Ser. No. 07/997,473, entitled "Ink Jet Printhead Having Compensation For Topographical Formations Developed During Fabrication", assigned to the same assignee as the present invention and filed on Dec. 28, 1992 now U.S. Pat. No. 5,412,412. The printhead enables better bonding of the two plates by compensating for raised lips or edges formed on the outside edge of opposing last pits in an array of pits located in the thick film layer that are created while photofabricating the pits in the insulating layer. The fabrication sequence compensates for the raised edges by including a non-functional straddling channel that nullifies the standoff created by the raised edge and a corresponding additional non-functional pit that positions the raised edge away from the functional channels and nozzles. Although this fabrication technique compensates for polyimide raised edges, it does not attempt to solve the problem of edge bead or dips between channels.
Another method of minimizing heater and channel plate standoff in ink jet printheads is disclosed in U.S. patent application Ser. No. 08/126,962, entitled "Ink Jet Printhead Which Avoids Effects of Unwanted Formations Developed During Fabrication", filed Sep. 27, 1993 now U.S. Pat. No. 5,450,108 and also assigned to the same assignee as the present invention. The minimization of standoff is obtained by sequentially patterning each layer of a two layer thick film layer. The relative thickness and geometrical shapes of the recesses in the two layers are selected, so that topographic formations are varied to prevent standoff between bonded heater and channel plates, thereby insuring that the adhesive applied between the bonded plates will have the greatest propensity to bond.
An article by P. Singer entitled "Chemical-Mechanical Polishing: A New Focus on Consumables," pages 48-52, Semiconductor International, February 1994, discloses planarization of integrated circuit devices on silicon wafers to less than 1 .mu.m by a process known as chemical-mechanical polishing. This process is not well understood, so that commercial production is difficult, when good planarity across the wafer, uniformity between wafers, and reliability is demanded, together with enough process latitude to prevent the polishing costs from being prohibitive. In a typical chemical-mechanical polishing process, the wafer is mounted on a rotatable carrier or chuck which is rotated and held down on a rotating polishing pad coated with a polishing slurry. The slurry typically consists of fumed silicon particles in an alkaline medium such as potassium or ammonium hydroxide. The polishing pad is generally made of cast or sliced polyurethane with a filler of urethane coated polyester felt. Pores in the pad surface aid in slurry transport, and the polymeric foam cell walls of the pad, in combination with the slurry particles, remove the reaction products from the wafer surface. Glazing of the pad's surface is thought to be the reason for the pad's drop in efficiency and removal rate over time. This means the pad surface must be reconditioned after every run by abrading its surface with, for example, a diamond wheel, thereby regenerating the surface rather than removing material from the pad.
The primary focus for chemical-mechanical polishing is to planarize continuous surfaces such as oxide passivation layers and continuous surfaces containing both oxides and metals. In contrast, the present invention is concerned with obtaining a planarized polyimide layer which has a discontinuous surface; i.e., one having recesses therein.
Article by R. Iscoff entitled "CMP Takes A Global View", pages 72-78, Semiconductor International, May 1993, discloses chemical-mechanical polishing (CMP) as the only viable means of globally planarizing patterned wafers with smaller than 0.35 .mu.m features. Because the technology is relatively young, the major equipment makers have not yet recognized CMP as a large market. The slurries for CMP offer much higher purity than older formulas which have been tailored for optical performance. Generally, though, it is not the slurries but the pads which are of most concern. They must be abrasive enough to planarize efficiently, but not too abrasive or they will damage circuits.
Article by S. Sivaram et al. entitled "Overview of Planarization by Mechanical Polishing of Interlevel Dielectrics", pages 606-614, ULSI Science and Technology, Electrochemical Society, 1991, discloses the need for extreme planarity in fine featured devices, and discloses that chemical-mechanical polishing is needed to obtain global planarity. Concepts behind material removal are extended to the polishing process and the chemistry of glass polishing is presented. The state of the art in the polishing technology is surveyed and the areas which need improvement are highlighted, so that the chemical-mechanical polishing process can be used in volume manufacturing.
Japanese Laid-Open No. 3-268392 (Kokai), published Nov. 29, 1991, discloses a manufacturing method for a multilayer interconnection or wiring board. A first wiring pattern is formed on an insulating substrate, together with cylindrical electroconductive columns connected thereto. The first wiring pattern and electroconductive columns are covered by an insulating layer. The surface of insulating layer is polished to planarize the insulating layer surface and to expose the electroconductive columns by a scanning polishing jig which has a polishing area smaller than 30% of the area of the wiring pattern. A second wiring pattern is formed on the flat insulating layer surface and connected to the exposed electroconductive columns.
U.S. Pat. No. 4,944,836 to Beyer et al. discloses a method for producing coplanar metal/insulator films on a substrate by chemical-mechanical polishing. In one example, a substrate having an insulating layer of dielectric material thereon is patterned to produce recesses therein and then the patterned insulating layer is coated with a layer of metal. The substrate is placed in a polisher and the metal is removed everywhere except in the recesses. This is made possible by the use of a selective slurry which removes the metal much faster than the dielectric material, thereby producing a continuous coplanar surface of metal and insulating material. In a second example, a substrate having a patterned metallic layer is coated with an insulating layer and then subjected to chemical-mechanical polishing. With an appropriate change in the slurry, the structure is coplanarized by the chemical-mechanical removal of the insulating material at a significantly higher rate than the underlying metal to be exposed at the termination of the polishing. The polishing pad is firm enough so that it does not deform under the polishing load. Thus, during the initial planarization action, the high points of the structure are removed at a faster rate than from the lower points.
There continues to exist, therefore, a need to prevent the standoff between either mated heater and channel plates or mated heater substrates with patterned barrier layers and nozzle plates caused by raised lips, wall sags or dips, and/or edge beads. Such standoff prevention is desired without requiring extra non-functional, straddling channels or in drastically altering the fabrication sequence of the heater and channel plates, as disclosed in the above-mentioned prior art.